Hum reducing system



Jan. 2, 1945. w 5. BACHMAN 366,471

HUM REDUCING SYSTEM Filed Oct. 10, 1942 inventor I William S. Bachman,

H Hig zbtorney.

Patented Jan. 2, 1945 HUM REDUCING SYSTEM William S. Bacliman, Fairfleld, Conm, assignor to General Electric Company, a corporation of New York Application October 10, 1942, Serial No. 461,569

3 Claims. (Cl. 179-171) well as to reduce distortion and to improve the frequency response characteristic. My present invention has for its object to effect certain further improvements with respect to the reduction of hum.

Another object 01' my invention is to combine certain hum reducing expedients with the degeneration means of my prior patent thereby to improve the operation of the amplifier with respect to hum while avoiding any material increase in cost of the receiver.

. The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. 'My invention risen, however, both as to its organization and method of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which the single figure represents an embodiment of my invention.

Referring to the drawing, I have indicated at I a transformer, which may be the last intermediate frequency transformer of a superheterodyne radio receiver Preferably the windings of this transformer are tuned to the intermediate frequency by condensers 2 and 3 respectively. The radio frequency voltage across the tuned secondary of this transformer is rectified by means of a diode comprising the cathode l of an electron discharge device 5 and an anode 6 arranged thereon. the tuned secondary being connected between this cathode 4 and diode 6 through a load resistor 1 shunted by a condenser 9. This resistor I may be the usual diode detector load resistance, and the condenser 8 the usual radio frequency bypass therefor, these elements being proportioned to have a time constant such that audio frequency voltages are produced across the resistance in response to audio frequency modulation of the rectified carrier wave.

The cathode 4 is grounded and comprises also the cathode of an audio frequency amplifier having an input electrode 8-and an anode 9 in the .to a loud speaker 2|.

discharge device 5. Audio voltage from the resistance l is supplied to the input electrode 8 by means of a potentiometer I0 having a variable contact I I, which is connected through a coupling capacitance I2 to the control electrode 8, the

latter being also connected to ground through the usual grid resistance IS. The potentiometer I0 is connected between the negative terminal of the resistance 1 and ground through a blocking condenser I4 and a small resistance I5, the latter being of the order of 1,000 ohms whereas the former, i. e., the volume control potentiometer I0, is of the order of two megohms.

The audio frequency amplifier comprises also an electron discharge device I9 having a pair of output terminals. The output voltage on' the anode 9 of amplifier 5 is supplied through coupling condenser I! to the input electrode of the subsequent amplifier I9 whereby it is amplified and supplied through an output transformer 20 One side of the secondary winding of the transformer 20 is grounded as indicated at 22 and its other terminal is connected through resistance 23 to a point between resistances I0 and I5. The secondary winding of transformer 20 is so poled that the voltage supplied therefrom across resistance I5 is supplied to the input electrode 8' of amplifier 5 in degenerative phase.

Operating voltage for the amplifiers is derived by rectification of a commercial source of alternating current, which may be obtained by means of a plug 25, which may be inserted in a conventional household wall outlet, for example. The alternatingecurrent voltage supplied therefrom is rectified by means of the diode 26 and,

is supplied through conductor 21 and the primary of the output transformer 20 to the anode of the amplifier I9. It is also supplied to the filter comprising resistance 28 and condensers 29 and 30, the latter comprising the usual filter for the reduction of the hum or ripplevoltages resulting from rectification of the alternatingcurrent voltage. Since the discharge device I9 requires higher operating voltage than does device 5 its anode voltage is derived from the input to the filter 28, 29, 30. The voltage at the output of the filter is supplied to the screen electrode of amplifier I9 and through resistance 3| to the anode 9 for operation of the amplifier 5.

Of course, owing-to the current of hum frequency, i. e., sixty cycles, and harmonics thereof, which -flows through resistance 3| and through the internal anode resistance of the discharge device 5 a hum voltage is developed on resistance ll, which is supplied through condenser II to the input of discharge device I9. Similarly, hum voltage due to the current of the hum frequency fiowing in conductor 21, the primary winding of transformer 20 and the anode resistance of discharge device I9 is also developed. The hum voltage supplied to the grid of this discharge device is so phased as to neutralize the eiiect of the latter in the transformer 20 thereby to minimize the production of hum currents in the circuit of the loud speaker 2|.

Of course, the constants of the system may be so proportioned that the hum voltage is materially reduced by the means thus described. A residuum of hum is, however, present. It has been found in accordance with my invention that this residuum of hum may be reduced in the system described and without any material additional cost of the system by connecting the resistance 35 between the output of the filter and the point between resistances l and I thereby to supply additional hum voltage to the degeneration resistance l5. This hum voltage is, of course, supplied to the input of the discharge device 5 just as is the feedback degenerative voltage from the output of the amplifier supplied to the input of discharge device 5 and it is poled so that its effect in the output of amplifier I9 is to reduce the residuum of hum previously referred to.

01' course, the amount of undesired hum voltage and degenerative voltage on resistance I5, which is supplied to the input of amplifier 5 is maximum when the contact I I is at the terminal of resistance It adjacent the upper terminal of resistance l5 and it is reduced as the contact II is moved upward. The resistance Ill, capacitance i4 and resistance 1 constitute a voltage dividing path across resistance It. Thus, as contact H is moved upward to increase the amount of signal voltage derived from resistance I, which is supplied to the amplifier, the amount of degenerative voltage and hum voltage on resistance l5, which is supplied to the amplifier is reduced. Resistances i5 and 35 are proportioned in such a ratio as to produce the minimum of hum when the contact Ii is at its lower position to which it is adjusted for minimum volume of sound reproduced by the loud speaker. As the contact H moves upward the hum, of course, increases. This, however, is not objectionable because the volume likewise increases and in an amount suificient to mask the effect of the hum. In this way the increased undesired hum sound reproduced by the loud speaker becomes unnoticeable.

. It has been found that by the use of my invention a very material reduction in hum is possible. In one equipment, by the connection of resistances l5 and 35, as described the hum was reduced from a level 01' -44 decibels to a level of 58 decibels or in a ratio of 5:1 reduction in voltage.

While I have shown a particular embodiment of my invention, it will, of course, be understood that I do not wish to be limited thereto since different modifications, both in the circuit arrangement and in the instrumentalities employed may be made and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a source of aud o voltage tions in said output produced by said operatingvoltage whereby as said variable portion of said resistance is increased said degeneration and neutralization effects are reduced and the volume of output is increased sufficiently to render unobjectionable any resultant undesired variations in said output.

2. In combination, a source of voltage to be amplified, a pair of resistances connected in series across said source, an electron discharge amplifier having input and output terminals, one input terminal being variabl connected along one of said resistors for volume control and th other input terminal being connected to the electrically remote terminal of the other resistor, means to supply voltage from said output terminals across said other resistor in degenerative phase, means to supply operating voltage subject to undesired variations to said amplifier and to supply said undesired variations across said other resistor in phase to neutralize said undesired variations produced between said output terminals by reason of said operating voltage, said resistances being proportioned to reduce said undesired variations in said output both by said neutralization and by degeneration when said one input terminal is in position for minimum volume, whereby when said variations are increased by movement of said terminal to increase volume the increased volume masks the'undesired increase in said undesired variations.

3. In combination, a pair of cascade connected electron discharge amplifiers having an output circuit, a power supply rectifier having an output hum filter, means to supply operating voltage from the input of said filter to the anode of the last of said amplifiers and from the output of said filter to the anode of the first of said amplifiers, whereby hum voltage supplied to said first of said amplifiers tends to neutralize in said output circuit the effect of hum voltage supplied to the last of aid amplifiers, a source of voltage to be amplified having a point connected to one input terminal of the first of said amplifiers, said point being variable on said source for volume control, and a resistance in series with said source, said resistance being connected across the output of the last of said amplifiers to have degenerative voltage applied thereto and being connected across the output of said filter to have hum voltage impressed thereon, said voltage being poled to neutralize any residuum of hum in the output of said last amplifier, said hum voltage and degenerative voltage being supplied through said source and variable input terminal to the first of said amplifiers in amounts variable with variation of said terminal on said source and being maximum when said terminal is positioned for minimum volume.

WILLIAM S. BACHMAN. 

